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<title>Bit-Fields (GNU Compiler Collection (GCC) Internals)</title>

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<h3 class="section" id="Bit_002dFields-1"><span>14.11 Bit-Fields<a class="copiable-link" href="#Bit_002dFields-1"> &para;</a></span></h3>
<a class="index-entry-id" id="index-bit_002dfields"></a>

<p>Special expression codes exist to represent bit-field instructions.
</p>
<dl class="table">
<dt><a class="index-entry-id" id="index-BITS_005fBIG_005fENDIAN_002c-effect-on-sign_005fextract"></a>
<a id="index-sign_005fextract"></a><span><code class="code">(sign_extract:<var class="var">m</var> <var class="var">loc</var> <var class="var">size</var> <var class="var">pos</var>)</code><a class="copiable-link" href="#index-sign_005fextract"> &para;</a></span></dt>
<dd><p>This represents a reference to a sign-extended bit-field contained or
starting in <var class="var">loc</var> (a memory or register reference).  The bit-field
is <var class="var">size</var> bits wide and starts at bit <var class="var">pos</var>.  The compilation
option <code class="code">BITS_BIG_ENDIAN</code> says which end of the memory unit
<var class="var">pos</var> counts from.
</p>
<p>If <var class="var">loc</var> is in memory, its mode must be a single-byte integer mode.
If <var class="var">loc</var> is in a register, the mode to use is specified by the
operand of the <code class="code">insv</code> or <code class="code">extv</code> pattern
(see <a class="pxref" href="Standard-Names.html">Standard Pattern Names For Generation</a>) and is usually a full-word integer mode,
which is the default if none is specified.
</p>
<p>The mode of <var class="var">pos</var> is machine-specific and is also specified
in the <code class="code">insv</code> or <code class="code">extv</code> pattern.
</p>
<p>The mode <var class="var">m</var> is the same as the mode that would be used for
<var class="var">loc</var> if it were a register.
</p>
<p>A <code class="code">sign_extract</code> cannot appear as an lvalue, or part thereof,
in RTL.
</p>
</dd>
<dt><a id="index-zero_005fextract"></a><span><code class="code">(zero_extract:<var class="var">m</var> <var class="var">loc</var> <var class="var">size</var> <var class="var">pos</var>)</code><a class="copiable-link" href="#index-zero_005fextract"> &para;</a></span></dt>
<dd><p>Like <code class="code">sign_extract</code> but refers to an unsigned or zero-extended
bit-field.  The same sequence of bits are extracted, but they
are filled to an entire word with zeros instead of by sign-extension.
</p>
<p>Unlike <code class="code">sign_extract</code>, this type of expressions can be lvalues
in RTL; they may appear on the left side of an assignment, indicating
insertion of a value into the specified bit-field.
</p></dd>
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